Reverse genetics systems / Novartis Ag

Title: Reverse genetics systems.Abstract: The invention provides various reverse genetics systems for producing segmented RNA viruses, wherein the systems do not require bacteria for propagation of all of their expression constructs. ...

BACKGROUND

Reverse genetics permits the recombinant expression and manipulation of viruses in cell culture. It is a powerful tool in virology and vaccine manufacture because it allows rapid production and/or mutation of viruses, including reassortant production. The method involves transfecting host cells with one or more plasmids which encode the viral genome then isolating (or “rescuing”) virus from the cells. It can be used for the production of a wide variety of RNA viruses, including positive-strand RNA viruses [1,2], negative-strand RNA viruses [3,4] and double-stranded RNA viruses [5].

A drawback of known methods is that they rely on plasmids. Generating these plasmids requires cloning steps to be performed in bacteria, which can take several days or weeks to perform and verify for a segmented RNA virus. Such delays interfere with the timetable for yearly production of seasonal influenza vaccines and also prevent a rapid response to a pandemic outbreak. Furthermore, the use of bacteria entails the risk that bacterial contaminants might be introduced when the plasmids are used to transfect a host cell for virus production. These drawbacks are addressed in reference 6 by using linear expression constructs instead of plasmids. The linear expression constructs do not contain amplification and/or selection sequences which are used during bacterial propagation and almost always results in the molecular cloning of a single representative of a viral quasispecies. Such linear expression constructs can be used to transfect host cells directly, giving a much more rapid reverse genetics system: reference 6 suggests that transfection of the linear constructs can be achieved within hours of receiving a viral isolate, avoiding the time required for molecular cloning and allowing access to useful members of the original viral quasispecies population.

DISCLOSURE OF THE INVENTION

For a segmented virus the method used in reference 6 uses one linear construct per viral segment. Thus reverse genetics virus production by this method requires transfection of a host cell with eight different constructs. An object of the invention is to avoid the need for such multiple transfections. More generally, it is an object of the invention to provide further and improved methods for practising reverse genetics for segmented RNA viruses, and in particular to provide further methods which do not require the use of bacteria. The invention provides various reverse genetics systems for producing segmented RNA viruses, wherein the systems do not require bacteria for propagation of all of their expression constructs. Ideally, bacteria are not required at all. producing segmented RNA viruses, wherein the systems do not require bacteria for propagation of all of their expression constructs. Ideally, bacteria are not required at all.

In a first aspect, a reverse genetics system is based on a non-bacterial expression construct which encodes at least two viral genome segments. This system reduces the number of constructs which have to be transfected into a host cell for production of a complete viral genome. For instance, a single construct can be used to encode eight influenza virus segments, thereby giving an 8-fold reduction in the complexity of transfections as compared to reference 6. Thus the invention provides a non-bacterial expression construct comprising coding sequences for expressing at least two different genome segments of a segmented RNA virus. The invention also provides a eukaryotic host cell including this non-bacterial expression construct. The invention also provides a set of two or more such non-bacterial expression constructs, wherein the set encodes a complete segmented RNA virus genome.

In a second aspect, a reverse genetics system is based on a combination of (i) at least one bacterial expression construct and (ii) at least one non-bacterial expression construct. Each of these two types of constructs provides at least one viral genome segment. Although this aspect does not totally avoid the use of bacteria for preparing the system, it is still powerful. For instance, constructs expressing a subset of the viral segments can be propagated and manipulated in bacteria, taking advantage of the wide range of convenient molecular biological techniques which are available. The segments of this subset can be those which do not often need to be changed from strain to strain. The remaining viral segments can be encoded by non-bacterial expression constructs, and these constructs can be rapidly prepared at short notice without requiring bacterial work. This combination thus means that efforts can focus on the segments of interest at short notice, and the constructs can be combined with an existing set of “background” segments which were already available. Thus the invention provides a set of expression constructs comprising (i) at least one plasmid comprising coding sequence(s) for one or more genome segments of a segmented RNA virus and (ii) at least one non-bacterial expression construct comprising coding sequence(s) for one or more genome segments of the RNA virus, wherein the combination of bacterial and non-bacterial constructs provides at least two different genome segments of the RNA virus. The invention also provides a eukaryotic host cell including this set of constructs.

In a third aspect, the invention provides a host cell including a linear expression construct which comprises coding sequences for at least two different genome segments of a segmented RNA virus. This cell may be bacterial but is preferably eukaryotic.

In a fourth aspect, the invention provides a bacterial plasmid comprising coding sequences for eight different genome segments of an influenza virus, wherein expression of each segment is controlled by either (i) a mammalian pol-I promoter or (ii) a bacteriophage polymerase promoter. The invention also provides a cell including this construct, and this cell may be bacterial or eukaryotic.

The invention further provides a process for preparing a host cell of the invention, comprising a step of inserting into the cell one or more expression construct(s) mentioned above.

The invention further provides a process for RNA expression in a eukaryotic host cell of the invention, comprising a step of culturing the host cell under conditions such that expression of the RNA virus segments occurs from the expression constructs.

The invention further provides a method for producing a segmented RNA virus, comprising a step of culturing a host cell of the invention under conditions such that expression of the RNA virus segments occurs from the expression constructs to produce the virus. Virus produced in this way may then be purified from the host cells or from a culture of the host cells. The invention also provides virus obtained by this process. This virus may be used to infect eggs or cells to grow virus for vaccine manufacture. Thus the invention provides a method for preparing a viral vaccine, comprising a step of infecting a culture host (e.g. eggs or cells) with a virus of the invention, growing the virus, and then preparing vaccine from the grown virus.

The invention also provides a process for preparing a DNA molecule which comprises coding sequences for expressing at least two different segments of a segmented RNA virus genome (e.g. a non-bacterial expression construct of the invention), wherein the DNA is prepared at least in part by chemical synthesis.

The invention also provides a process for preparing a DNA molecule which comprises coding sequences for expressing at least two different segments of a segmented RNA virus genome (e.g. a non-bacterial expression construct of the invention), wherein the process comprises steps of: (i) synthesising a plurality of overlapping fragments of the DNA molecule, wherein the overlapping fragments span the complete DNA molecule; and (ii) joining the fragments to provide the DNA molecule. The DNA molecule may then be recovered and used in the reverse genetics methods of the invention e.g. it can be inserted into a eukaryotic cell for generation of the segmented RNA virus. Preferably the DNA molecule is not inserted into a bacterial cell between its recovery and its insertion into the eukaryotic cell i.e. the construct is used directly for viral rescue without any intermediate bacterial amplification.

The invention also provides a library of expression constructs for a segmented RNA virus, wherein each expression construct comprises a coding sequence for at least one genome segment of the virus. The library includes at least one construct for each segment of the genome, such that the whole genome can be represented by selecting a subset of the library. Some viral segments may be represented more frequently than others e.g. an influenza virus library may include many more HA and NA segments than the average. To construct a desired viral genome of interest, library members encoding each desired segment are selected and then expressed to give the desired virus. The library is particularly powerful for influenza virus by permitting rapid reassortment of backbone genome segments with HA and NA segments of interest to produce a useful virus for vaccine production.

Non-Bacterial Expression Constructs

The first, second and third aspects of the invention utilise one or more “non-bacterial expression constructs”. This term means that the construct can drive expression in a eukaryotic cell of viral RNA segments encoded therein, but it does not include components which would be required for propagation of the construct in bacteria. Thus the construct will not include a bacterial origin of replication (ori), and usually will not include a bacterial selection marker (e.g. an antibiotic resistance marker). These components are not required for driving the desired viral RNA expression in a eukaryotic host cell and so are superfluous when bacteria are not used for propagation of the constructs. Absence of these propagation components means that the constructs will not be replicated if they are introduced into bacteria.

The non-bacterial construct may be linear or circular. Linear constructs are more usual (as seen in reference 6), but circular constructs can also be used. Circular constructs can be made by circularising linear constructs and vice versa. Methods for such circularisation are described in ref. 6. Linearisation of a circular construct can be achieved in various easy ways e.g. utilising one or more restriction enzyme(s), or by amplification from a template (including a circular template) using a nucleic acid amplification technique (e.g. by PCR).

A non-bacterial construct includes coding sequence(s) for one or more viral RNA segment(s). Constructs for the first and third aspects encode at least two different viral RNA segments. The encoded segments can be expressed and then function as viral RNAs which can be packaged into virions to give recombinantly expressed virus. Thus the constructs are suitable for producing a RNA virus by reverse genetics, either alone or in combination with other constructs.

The construct will usually be made of double-stranded DNA. Such constructs can conveniently be made by known methods of DNA synthesis and assembly. Modern techniques can provide synthetic DNA molecules encoding a complete virus even if it has many genomic segments. For example, a construct expressing all eight segments of the influenza virus genome requires about 25,000 base pairs (25 kbp) of DNA, which is well within the capability of current construct synthesis e.g. reference 7 reports chemical synthesis of a 32 kbp gene by assembly of individual ˜5 kbp synthetic fragments, and reference 8 reports the production of a 583 kbp synthetic chromosome via intermediate stages of about 5 kbp, 7 kbp, 24 kbp, 72 kbp or 144 kbp long. See below for further details.

Such synthetic methods are the preferred way of providing constructs (and in particular of providing linear constructs). Instead of using chemical synthesis, however, DNA for a construct can be prepared from a RNA virus by reverse transcription to provide a cDNA, and extra DNA sequences can then be joined to the cDNA (e.g. by ligation) or the cDNA can be incorporated into a larger DNA construct. In some embodiments, a mixture of enzymatic and chemical methods is used e.g. reverse transcription followed by chemical addition to the termini.

As well as being free from any bacterial propagation elements, the non-bacterial construct may also be free from any bacterial DNA modifications. Thus the construct may include no methylated adenine residues, and any methylated cytosine residues will be in the context of a CpG dinucleotide motif i.e. there will be no methylated cytosines which are not followed by a guanidine.

The construct can be introduced into a host cell by any suitable transfection method e.g. by electroporation, lipofection, DEAE-dextran, calcium phosphate precipitation, liposomes, gene guns, microparticle bombardment or microinjection. Once transfected, the host cell will recognise genetic elements in the construct and will begin to express the encoded viral RNA segments.

Construct Synthesis

As mentioned above, a DNA expression construct may be prepared by chemical synthesis at least in part. The construct comprises coding sequences for expressing at least two different segments of a segmented RNA virus genome (and preferably for expressing the complete genome of a segmented RNA virus) and can conveniently be prepared using the synthetic methods disclosed in reference 8.

The synthetic method may involve notionally splitting the desired DNA sequence into fragments. These fragments may again be notionally split one or more times, eventually arriving at a set of fragments which are each of a size which can be prepared by a chosen DNA synthesis method e.g. by phosphoramidite chemistry. These fragments are then synthesised and joined to give the longer fragments from the notional splitting stage, and these longer fragments are then joined, etc. until the complete sequence is eventually prepared. In this way reference 8 prepared a 583 kbp genome by assembling 18 104 50 mer oligonucleotides in various stages. The 50 mers were assembled into cassettes 5-7 kb long, and these cassettes were then assembled into ˜24 kbp fragments, which were then assembled into ˜72 kbp fragments, then ˜144 kbp, then giving two ˜290 kbp constructs, which were finally joined to give the complete genome.

The fragments are designed to overlap, thereby permitting them to assemble in the correct order. For instance, the cassettes overlapped by at least 80 bp, thereby enabling their assembly into the ˜24 kbp fragments, etc. Thus the method involves the synthesis of a plurality of overlapping fragments of the desired DNA molecule, such that the overlapping fragments span the complete DNA molecule. Both ends of each fragment overlap with a neighbouring 5′ or 3′ fragment, except for the terminal fragments of a linear molecule where no overlap is required (but to synthesise a circular molecule, the two terminal fragments should overlap). Fragments at each stage may be maintained as inserts in vectors e.g. in plasmids or BAC or YAC vectors. Assembly of fragments during the synthetic process can involve in vitro and/or in vivo recombination. For in vitro methods, digestion with a 3′ exonuclease can be used to expose overhangs at the terminus of a fragment, and complementary overhangs in overlapping fragments can then be annealed, followed by joint repair (“chewback assembly”). For in vivo methods, overlapping clones can be assembled using e.g. the TAR cloning method disclosed in reference 8. For fragments <100 kbp (e.g. easily enough to encode all segments of an influenza virus genome) it is readily possible to rely solely on in vitro recombination methods.

Other synthetic methods may be used. For instance, reference 7 discloses a method in which fragments ˜5 kbp are synthesised and then assembled into longer sequences by conventional cloning methods. Unpurified 40 base synthetic oligonucleotides are built into 500-800-bp synthons by automated PCR-based gene synthesis, and these synthons joined into multisynthon ˜5 kbp segments using a small number of endonucleases and “ligation by selection.” These large segments can be subsequently assembled into longer sequences by conventional cloning. This method can readily provide a 32 kbp DNA molecule, which is easily enough to encode a complete influenza virus.

Similarly, reference 9 discloses a method where a 32 kb molecule was assembled from seven DNA fragments which spanned the complete sequence. The ends of the seven DNAs were engineered with unique junctions, thereby permitting assembly only of adjacent fragments. The interconnecting restriction site junctions at the ends of each DNA are systematically removed assembly.

Once the complete DNA molecule has been assembled, it is purified and may be inserted directly into eukaryotic cells for virus production, without involving an intermediate step where the DNA is present inside a bacterium.

When prepared by these methods, a DNA expression construct of the invention may include one or more “watermark” sequences. These are sequences which can be used to identify or encode information in the DNA. It can be in either noncoding or coding sequences. Most commonly, it encodes information within coding sequences without altering the amino acid sequences. For DNAs encoding segmented RNA viral genomes, any watermark sequences are ideally included in intergenic sites because synonymous codon changes may have substantial biological effects for encoded RNA segments.

Plasmids

The second and fourth aspects of the invention involve the use of plasmids. These plasmids can conveniently be propagated in bacteria and so include a bacterial origin of replication (ori) and usually also include a bacterial selection marker (e.g. an antibiotic resistance marker). Thus the plasmids are readily distinguished (both by sequence and by function) from the non-bacterial expression constructs discussed above. In general terms, the plasmids may be the same as plasmids already known in the art for reverse genetics, but the prior art does not disclose their use in combination with non-bacterial expression constructs for virus rescue.

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20121025|20120270321|reverse genetics systems|The invention provides various reverse genetics systems for producing segmented RNA viruses, wherein the systems do not require bacteria for propagation of all of their expression constructs. |Novartis-Ag